Surface Of Soil Particles Research Articles

Is It Possible to Use 90Sr and 137Cs As Tracers for the Aeolian Dust Transport

A noble approach for the estimation of the deposition of the Asian dust over Japan by using atmospheric anthropogenic radioactivity, 90Sr and 137Cs, is proposed. Those radioactivities once deposited globally onto the ground by the past nuclear test, etc. are resuspended along with the surface soil particles in the air by the storm and deposited again on the ground. The difference in 137Cs/90Sr activity ratios in the surface soil between dry and wet climatic conditions gives a clue to know about the source of the dust. It was found that the average activity ratio in the deposited material (average: 2.1, n=82) at MRI, Tsukuba in the 1990s is not close to that of the surface soil taken in neighborhood (median: 6.8, n=8), which had been considered as the primary source. While, it is close to that of the soil samples taken in the arid area in the Asian continent (median: 2.0, n=4), which is the one of the source regions of the aeolian dust. Thus we can reasonably assume that deposited dust in Tsukuba is brought not only from the neighboring fields, etc. but also from the arid area in the continent. Using a simple two-component model it was estimated that the Asian dust may transport about 90% of 90Sr and 70% of 137Cs depositions observed in Tsukuba in the 1990s. Also, Asian dust may transport about 2/3 of the dust deposited in Tsukuba.

Laser desorption/ionization coupled to tandem mass spectrometry for real-time monitoring of paraquat on the surface of environmental particles.

Aerosol mass spectrometry with laser desorption/ionization was investigated as a possible tool for real-time monitoring of the presence of the pesticide paraquat on the surface of airborne soil particles. Laser desorption/ionization of paraquat dication produced only singly charged ions. The most abundant species were [M](+.), [M - H](+), and [M - CH3](+). Operation of the ion trap mass spectrometer in the MS(3) mode allowed the reduction of the signal dependence on laser fluence fluctuations and permitted the detection of the analyte with good sensitivity and high selectivity. The estimated limit of detection in terms of surface coverage was 0.016 monolayers, approximately 1 attomole of paraquat on the surface of a single micron-sized soil particle.

A New Explanation for the Behavior of Saturated Soils Subject to Vigorous Vibration

The high effectiveness of the deep compaction of saturated soils by percussive loads (heavy tampers , piles, deep-seated blasting), the cause of which had not been explained until now, is a basic peculiarity o f their behavior . Much information about this problem is cited in [1] . The effectiveness of the percussive compaction of gas-containin g soils has never been in doubt. As fo r the compaction of saturated soils, many soil engineers have referred to use of this method for some time wit h skepticism, and have placed the validity of the results obtained in doubt. This is explained by the existin g notion of a saturated soil as a medium that is essentially incompressible under short-lived loads. Specialists consider that the compaction of saturated soils should be accompanied by the pressing o f water from the pores in the soil. Moreover, the pressing is represented as the compressive squeezing of a sa t urated soil due to squeezing of the water-free and air-filled pore space, which usually amounts to 1-3% of th e volume of the pores (internal pressing), and also as an outflow of fluid through drainage interlayers into th e peripheral sections of the soil being compacted (external pressing). The hypothesis of soil liquefaction, which consists in the fact that the soil is liquefied each time in th e impact phase, and is consolidated in the at-rest phase, was proposed to explain the mechanism responsible fo r the consolidation of soil during its compaction by repeated impacts. After comprehensive analysis, Rabinovic h and Khavin [1] drew the following conclusions . The high effectiveness of the compaction of saturated soils by percussive loads can be explained, if w e adopt the acoustic cavitation that develops in the pore water under vigorous vibration as an intermediate lin k in the formulation of an opinion . Acoustic cavitation (a short-lived pressure pulse to 1 0 3 MPa and higher, and the heating of gas in bu b bles to 10°C) is the reason that the pore water is excited, and electromolecular interactions between the wate r molecules and the surface of the soil particles are disrupted one with the other. As a result, all pore wate r becomes free water and is less viscous; as compared with what one would expect, this appreciably increase s the water’s mobility in the soil. This, in turn, increases the volume of externally pressed water, which co n tributes to the soil’s compaction . The cavitation initiates and accelerates many physico-chemical reactions [2], which in some manner ,

A long-term soil leaching column experiment investigating the effect of variable sulphate loads on soil solution and soil drainage chemistry

A series of leaching column experiments were set-up to investigate the effects of increasing and decreasing the sulphate load on the uppermost mineral horizon of an acidified podzolic soil from NE Scotland. The soils showed signs of recovery when the sulphate load was reduced and acidified further when the sulphate load was increased. For the soils in which simulated precipitation inputs were less acidic than the soil pH, cation adsorption was still occurring at the end of the experiment. Adsorption was probably limited by the rate of diffusion of ions from the bulk soil water to the surfaces of soil particles. The leachate and soil solution from the experiment in which the least acidic input was applied were more acidic than the equivalent solutions from the experiment with the intermediate pH input. This was due to the acidifying effect of the desorption of protons from soil particles. The leachate and soil solution chemistry and the weathering rate of the soil subject to the most acidic simulated precipitation were modelled using the PROFILE model. The predicted bulk weathering rate was over estimated by a factor of 1.2. The predicted weathering rates of individual ions varied from the measured values by factors of between 0.06 and 10. Both the measured and PROFILE predicted weathering rates fall in the critical load of acidity class suggested for the soil on the basis of soil parent material by the Skokloster classification. PROFILE failed to predict accurately the measured soil solution and leachate chemistry.

Assessing the surface composition of soil particles from some Podzolic soils by X-ray photoelectron spectroscopy

We have applied X-ray photoelectron spectroscopy (XPS) to assess the surface composition of soil particles in the AB, Bf, and Bhf horizons of some Podzolic soils from British Columbia, Canada. Since the analysis is restricted to a `depth' of <10 nm from the particle surface, this technique is uniquely suited to this purpose. For the soils used here, the elements detected in decreasing order of abundance are oxygen (46–54%), carbon (13–38%), silicon (3.2–20%), aluminium (4.8–14%), iron (1.0–4.4%), and nitrogen (0.4–2.9%). Small amounts of sodium and magnesium are also measured. In comparison with the corresponding values for the bulk soils, the surface of soil particles is enriched in C, N, and Al. As C and N may be assigned to organic matter (OM), this finding is strongly indicative of the presence of Al–organic complexes as a coating over soil particle surfaces. Unlike Al, Fe is relatively depleted at the surface of soil particles probably because Fe tends to precipitate as an oxyhydroxide phase in the pH range (4.1–5.3) of the soils sampled. Silicon is also depleted at soil particle surfaces but the extent of depletion is smaller for the AB than for the underlying Bf horizons.

Boron sorption by manganese oxide‐coated sand

Manganese (Mn) in soils occurs mainly as hydrous oxides in the form of nodules, concretions or coatings on the surface of soil particles playing an important role in the retention of nutrients or heavy metals. Boron (B) is one of these elements found to be fixed by Mn‐oxides in a relatively available form. This research was conducted to study B sorption by Mn‐oxides prepared in the laboratory in relation to soil pH. Two Mn‐oxides were prepared as coatings on the surface of quartz sand, pyrolusite and birnessite. Boron sorption experiments carried out with uncoated sand (US), pyrolusite‐coated sand (PS), and birnessite‐coating sand (BS) showed that B sorption was increased in PS and BS in comparison to US. Total B sorption was greater on BS than PS and BS sorbed more unrecoverable B by dilute hydrochloric acid method. In US and PS, B sorption was linearly increased with pH. In BS, maximum B sorption occurred at pH 8.5. The operating mechanisms seems to be H bonds between boric acid and OH groups in pyrolusite and lingand exchange between borate and surface hydroxyls in birnessite.

Cadmium and zinc in acid tropical soils: I. Soil physico‐chemical properties effect on their adsorption

Interactions of heavy metals, such as cadmium (Cd) and zinc (Zn) at the soil particle surfaces play an important role in controlling their leaching losses to the underground water and their availability to plants. Adsorption isotherms for eight soils [Apas (Rhodic Hapludox), Batang (Typic Plinthudult), Jarangan (Xanthic Hapludox), Katai (Typic Hapludult), Koyah (Oxic Dystropept), Lumisir (Typic Plinthudult), Paliu (Typic Hapludult), and Table (Typic Hapludox)] from major cocoa (Theobroma cacao L.), oil palm (Elaeis guineensis Jacq.), and rubber (Hevea brasiliensis Muell Agr.) growing areas in Tawau, Sabah, Malaysia were determined at concentrations of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mg Cd or Zn mL‐1 in a 0.025M calcium chloride (CaCl2‐2H2O) solution. The adsorption of Cd and Zn by the soils can successfully be explained by the Freundlich equation (r = 0.832** ‐ 0.977**) as compared to the Langmuir equation (r = 0.130ns ‐ 0.977**). The ability of the soils to adsorb Cd and Zn differed markedly with the highest obtained in Table soil (18.4 mg Cd kg‐1 soil) for Cd and Koyah soil (29.97 mg Zn kg‐1) for Zn. The important soil parameters in controlling the adsorption of both Cd and Zn in acid tropical soils were cation exchange capacity (CEC) (r = 0.888**), total aluminium (Al) content (r = 0.675**), and extractable‐Al (r = 0.875**).

Transformation and retention of nitrogen in a coastal forest ecosystem

Transformations and fluxes of N were examined in three forested sites located along a gradient of soil texture in the coastal forests of the Waquoit Bay watershed on Cape Cod. Total N leaching losses to ground water were 0.5 kg ha-1 yr-1 in the loamy sand site and 1.5 kg ha-1 yr-1 in the fine sand site. Leaching loss to groundwater was not measured in the coarse sand site due to the prohibitive depth of the water table but total N leaching loss to 1m depth in the mineral soil was 3.9 kg ha-1 yr-1. DON accounted for most of the leaching losses below the rooting zone (77–89%) and through the soil profile to ground water (60%–80%). Differences in DON retention capacity of the mineral soil in the sites along the soil texture gradient were most likely related to changes in mineral soil particle surface area and percolation rates associated with soil texture. Forests of the watershed functioned as a sink for inorganic N deposited on the surface of the watershed in wet and dry deposition but a source of dissolved organic N to ground water and adjoining coastal ecosystems.

ADSORPTION EQUILIBRIUM OF CARBON TETRACHLORIDE ON DRY SOILS*

Vapor phase adsorption equilibrium of carbon tetrachloride, a priority pollutant, on dry soils was studied at 288, 293, and 298 K. Using a gravimetric adsorption apparatus, adsorption /desorption isotherms of carbon tetrachloride were generated on two different soil samples. The effects of temperature and soil characteristics were examined. Isosteric heals of adsorption were calculated and heat curves were constructed. Adsorption isotherms of carbon tetrachloride on dry soil samples were Type II, indicating formation of multilayers of adsorbate on the soil particle surface. Considerable hysteresis effects, associated with capillary condensation, were observed upon desorption. Thermal data confirmed that the adsorption of carbon tetrachloride vapor on soil was primarily physical adsorption. Heat curves showed that the soil samples had energetically heterogeneous surfaces. A positive correlation between The soil's specific surface area and its sorption capacity was observed. Clay content and pore size were also dominating factors. The experimental data were correlated by the Polanyi Potential, the BET, and the GAB models in order to provide input lo fate and transport models predicting the degradation or movement of volatile organic pollutants in soil. The BET equation gave accurate data fit, within a deviation range of 2·63-5·40%, for up to 40% of the saturation pressure. The GAB equation provided superior fit of the data for the entire relative pressure range. Absolute error percentages from the GAB model ranged from 1·77 to 5·38%. Results followed the Potential Theory satisfactorily and led to a single temperature-independent characteristic curve.

Physicochemical soil-contaminant interactions during electrokinetic extraction

The feasibility of using electrokinetics to extract contaminants from soils has been established by bench-scale laboratory experiments and small-scale field tests. However, the physics and chemistry associated with the innovative remediation technology are not yet fully understood. Many physicochemical reactions occur simultaneously during the process. These reactions may enhance or reduce the cleanup efficiency of the process. They are particularly important in fine-grained soils because the large specific surface area of the soil provides numerous active sites for these reactions. In this paper, several prominent physicochemical soil-contaminant interactions during electrokinetic extraction and their influences on the cleanup efficiency of the technology are discussed. These interactions include: (1) change of zeta potential at the soil particle/pore fluid interface; (2) resistance of the soil-fluid-contaminant system to pH change; and (3) sorption/desorption of reactive contaminants onto or from the soil particle surface and precipitation/dissolution of metallic contaminants in the pore fluid. The effects on these interactions of injecting an enhancement fluid into the contaminated soil are also discussed. In addition, a brief review on the state-of-development of the technology is presented.

Incomplete Hydrocarbon Biodegradation in Contaminated Soils: Limitations in Bioavailability or Inherent Recalcitrance?

Bioremediation has been used to treat soils contaminated with complex mixtures of organic compounds such as total petroleum hydrocarbons (TPH), oil and grease (O&G), or polycyclic aromatic hydrocarbons (PAHs). Despite the common use and cost-effectiveness of bioremediation for treating hydrocarbon-contaminated soils, it has been observed that a residual fraction remains undegraded in the soil even when optimal biodegradation conditions have been provided. This paper provides a brief review of the two major conceptual models that have been used to explain why a residual hydrocarbon fraction remains in the soil after bioremediation treatment. The contaminant sequestration model is based on the assumption that a certain fraction of hydrocarbons is “locked up” in small soil pores within soil particles or aggregates. These sorbed hydrocarbons are believed to be inaccessible to soil microorganisms. Consequently, limitations in bioavailability are thought to be the major reason for incomplete hydrocarbon biodegradation, particularly in aged or weathered soils. Alternatively, according to the inherent recalcitrance model, incomplete TPH biodegradation may be caused by the presence of certain hydrocarbons that are inherently recalcitrant to biodegradation or are only extremely slowly degradable even under optimal conditions. Each conceptual model provides different explanations regarding the potential risks of the residual hydrocarbon fraction. If the residual TPH is truly sequestered within the soil pore space, it is unlikely that these compounds will pose any significant risk to human or environmental receptors. By contrast, these risks may be considerably greater if the residual TPH fraction consists of inherently recalcitrant compounds that reside mostly on the surface of soil particles and therefore are much more available to potential receptors. Both conceptual models and their implications for the potential risk of the residual TPH fraction are discussed.

A numerical model of heat and water movement in furrow‐sown water repellent sandy soils

Water repellent soils have thin layers of hydrophobic organic matter on the surfaces of soil particles. The hydrophobic organic matter makes these soils difficult to wet in opening rains after dry periods. This can cause losses of crop and pasture production and encourage land degradation by wind and water erosion. Wetting patterns in water repellent soils are irregular and incomplete, and no successful model of heat and water movement in water repellent soils has been reported. Furrow sowing improves crop production on water repellent sandy soils. Seeding machinery forms ridges and furrows. Rain is easily shed from dry ridges and runoff accumulates in the furrow (water harvesting). Ponded water infiltrates easily into the soil around the seed. The water movement in furrow‐sown water repellent sandy soils is more regular. This regularity makes it possible to develop a model. A numerical model of heat and water movement in furrow‐sown water repellent sand was developed and validated to help optimize the design of furrow sowing. The model used aerodynamic resistance, soil surface moisture and temperature, and surface evaporative resistance to estimate evaporation from each surface element. Wind speed and radiation was varied by calculation, according to the direction of the Sun or wind to the ridge direction and height. Movement of water and heat in the soil was determined by the initial conditions, boundary conditions, and coupled functions for heat and water movement, as used in many other models. Water repellency effects on drying were included by including the appropriate moisture and unsaturated hydraulic conductivity characteristics. Application of the finite element method to a two‐dimensional cross section of a ridge and furrow system allows water to be excluded from a soil region beneath a ridge. Field experiments were used to estimate surface evaporative resistance characteristics and to validate the model. The model has been successfully used to suggest optimized designs for ridge and furrow systems to minimize high soil temperatures and evaporation in Western Australia.

Radium and uranium concentrations in two eskers with enhanced radon emission

Concentrations of 226Ra and 238U in soil samples from two Swedish eskers were determined. One of the eskers, containing rock material from a radioactive granite, revealed a high degree of radioactive disequilibrium between 226Ra and 238U. The disequilibrium ratio was positively correlated with particle size and soil permeability, indicating that a major part of the 226Ra is adsorbed on the surface of the mineral particles. The other esker, with uraniferous alum shale material and carboniferous slates, revealed a much lower degree of disequilibrium, indicating that a major part of the 226Ra is bound to primary uranium minerals. A less intense weathering of the uranium minerals due to the protective effect from the kerogen compounds in the alum shale is thought to be the main reason for this. The results indicate that 226Ra may be present in a weathered soil in two ways: in primary uranium minerals or precipitated on the surface of the mineral particles. The proportions may vary depending on the type of rock material, weathering conditions, and permeability of the soil. This is of great importance to the process of radon emanation, since it is much higher from radium that is adsorbed on the surface of soil particles compared to radium bound in primary uranium minerals.

Fundamental formulation of electrokinetic extraction of contaminants from soil

Electro-osmosis and ionic migration are the basic cleanup mechanisms in the electrokinetic extraction of contaminants from fine-grained soils. These are coupled flows as the flows of fluid and contaminants are driven by an externally applied electrical gradient. Moreover, other electrochemical reactions will occur simultaneously during the process. The most pronounced effect is the generation of pH gradient in the soil. The change of pH in the pore fluid can have a significant impact on the degree of sorption and desorption of chemicals on soil particle surfaces, complexes formation and precipitation of chemical species, and dissociation of organic acids; thus affecting the feasibility and efficiency of the cleanup technique tremendously. An attempt is made to formulate the coupled flows of ionic contaminants and the resulting change of pH in the pore fluid during the electrokinetic extraction process. The coupled flows of contaminants are formulated by the formalism of nonequilibrium thermodynamics. The pH is determined as a function of time and space by maintaining electrical neutrality throughout the system all the times. A numerical model NEUTRAL is developed to simulate the processes. The good agreement between computed and experimental results published in the literature indicates that the approach is a valid step toward a better understanding of the physics and chemistry involved during electrokinetic treatment of contaminated soils. Key words : electrokinetics, in situ remediation, contaminated soil, coupled flows, electrochemistry, nonequilibrium thermodynamics.

Origins and concentrations of 222Rn, 210Pb, 210Bi and 210Po in the surface air at Lisbon, Portugal, at the Atlantic edge of the European continental landmass

Concentrations of 222Rn, 210Pb, 210Bi and 210Po were measured in surface air at Sacavem, near Lisbon, at the junction of the North Atlantic ocean and the European continent. 222Rn concentrations were strongly associated with the origin of the air masses, being very low, ∼0.1 Bq m −3, in maritime air and much higher, 2–10 Bq m −3, in continental air masses. Furthermore, different 222Rn: 210Pb activity ratios were found in those air masses with average values of 1 × 10 3 and 5 × 10 4, respectively. In the short time intervals of sampling, 210Pb concentrations in the atmosphere were positively correlated with aerosol concentrations, suggesting soil resuspension as 210Pb source. However, comparison of 210Pb: 238U activity ratios in aerosols and surface soil particles provided evidence that resuspensions give little contribution, averaging 10%, to 210Pb in surface air. A seasonal fluctuation in the concentration of 210Pb in surface air was observed being higher 210Pb concentrations consistently measured during fall and winter. Nevertheless, concentrations of 210Pb in surface air were not controlled by the seasonal rainfall. Instead, the seasonal fluctuation of 210Pb corresponds to the seasonal influence on the region of air masses with different origins: predominantly, of continental origin in fall and winter and of maritime origin during spring and summer. The annual average of 210Pb at Sacavem, 181 μBq m −3, is substantially lower than values reported for inland locations, attesting for the strong influence of oceanic air masses. The mean residence time (MRT) of aerosols, 3.8 d, was found comparable to values reported for other regions, but data further suggest different MRTs with the seasons of the year. It is concluded that at Sacavem, the concentrations of radon and radon daughters, particularly those of 210Pb, are controlled mainly by large-scale meteorological conditions, especially through the alternate intrusion of either oceanic or continental air masses into the local surface air. 222Rn: 210Pb concentration ratios in those air masses are distinct and provide a means to identify their origin.

The Effect of Moisture and Soil Texture on the Adsorption of Organic Vapors

AbstractThe fate of volatile organic compounds (VOCs) moving as vapors in the subsurface is dependent on their interaction with the soil. Adsorption of VOC vapors is greatly influenced by soil texture and soil‐water content. The effects of differences in texture and soil‐water content on vapor partition coefficients for trichloroethylene (TCE) were examined. Batch experiments were conducted for a variety of soils and at different soil‐water contents, w, to determine the relationship between the vapor/solid partition coefficient, KD′, and w. In dry soils, KD′ was nonlinearly related to soil‐water content because, in that range, water molecules compete with VOC molecules for adsorption sites on the soil surface. Under wet conditions, KD′ became linearly related to water content according to Henry's law, indicating that adsorbed water molecules were acting as a solvent for VOC molecules. In general, KD′ under oven‐dry conditions did not relate well to total specific surface area of soils, most likely because VOC molecules adsorb only on the outside surfaces of soil particles (due to their nonpolarity), rather than the total surface area present. In the dry range, adsorption was dominated by soils with high specific areas (i.e., high clay content), while soils with higher organic carbon content manifested higher adsorption amounts in the wet moisture range. A one‐parameter, exponential model well described the log KD′‐w curve in the nonlinear region. The model parameter, a, was found to be highly dependent on the specific surface area of the soil. The proposed KD′‐(w) model incorporated in conventional VOC transport models seems promising for analyzing the effects of VOC vapor adsorption on VOC subsurface transport.

Parameters of Surface Heterogeneity from Laboratory Experiments on Soil Degradation

AbstractIn this study, we examined the applicability of the theory of gas adsorption on heterogeneous surfaces to assess changes in soil particle surfaces in degrading soils. Soil degradation was simulated by laboratory treatments of gray forest soil (Udic Argiboroll), chernozem soil (Typic Haploboroll), and chestnut soil (Ustolic Orthid) samples. Water vapor adsorption data were used to estimate the frequency distribution of surface adsorption energy and to assess the strength of the surface attractive field. A modified Brunauer‐Emmett‐Teller equation provided a good fit of the water vapor adsorption isotherms on soil minerals for relative vapor pressures in the range of 0.05 to 0.85. This equation was used as a local isotherm equation. The piecewise distribution technique was applied to estimate the surface adsorption energy distribution. After a H2O2 treatment, the monolayer capacity and average adsorption energy decreased in the samples of all three soils. After cyclic wetting‐drying, the adsorption energy range decreased. After silica acid treatments, the monolayer capacity decreased, and the energy distribution became much narrower. All treatments resulted in a loss of the low‐energy adsorption sites and an increase in the number of layers retained by the surface at saturation vapor pressure. The exchangeable Ca content changed after the treatments in the same way as the monolayer capacity did. The exchangeable Mg content increased after organic matter oxidation and slightly decreased after the silica treatments. The exchangeable K content increased after both organic matter oxidation and the wetting‐drying treatments.

Chromium leaching behavior in soil derived from chromite ore processing waste

To obtain information for the remediation design of chromium waste sites, the leaching behavior of chromium in chromium-contaminated soil (Cr-soil) derived from chromium ore processing residue (COPR) was investigated. Batch leaching experiments were conducted using simulated rainwater as the leaching solution with pH adjusted to cover a range from 2.0 to 12.0 No Cr(VI) was detected in the leachate at low pH (<2.5). This may be attributed to adsorption of Cr(VI) onto the soil surface and/or reduction of Cr(VI) to Cr(III) by organic matter and/or by ferrous iron (two other major components of the soil); these processes are favored at low pH. Significant amounts of Cr(VI) were leached between pH 4.5 and 12. Results from leaching experiments indicate that approximately 1% of total Cr (26 mg/g) is readily leachable. The major chromium form in Cr-soil was identified as chromite using X-ray diffraction analysis (XRD). The forms of leachable chromium cannot be identified. It can only be hypothesized that Cr(VI) is leached by a dissolution of the chromate salts and attenuated by adsorption/desorption, precipitation, and redox processes that may occur in the soil-water system. Removal of the organic matter from the Cr-soil increases the amount of Cr(VI) leached over the entire pH range, suggesting that the organic matter can reduce Cr(VI) present in the solution. Cr(III) leaching behavior was also investigated as a function of pH. Cr(III) was found in solution at pH < 5. The amount of Cr(III) leached was controlled by the solubility of Cr(III) precipitates, the extent of Cr(VI) reduction, and the magnitude of Cr(III) adsorption onto the soil surface. No Cr(III) was detected between pH 4.5 and 12 which can be attributed to the presence of insoluble precipitates such as Cr(OH) 3(s) and Cr xFe 1−x(OH) 3(s) and the adsorption of Cr(III) species onto the soil particle surface.

Adsorption of pentachlorophenol by natural soils

Abstract The adsorption of pentachlorophenol, a listed hazardous substance, by natural soils was investigated at pH values of 4, 7, and 10 using various soils collected from the surface horizon. The effects of soil properties, such as organic carbon and fines content, were also studied and were found to have a significant positive correlation with the amount of solute adsorbed. Although soil properties strongly influence the process, the pH of the soil‐water system was found to be a primary factor which determines the affinity of solute for the soil particle surface. Pentachlorophenol, a weak organic acid, is transformed to pentachlorophenate ion at pH higher than pKa and adsorption is reduced. The anionic form of the molecule was found to be much more hydrophilic than the neutral form resulting in a two orders of magnitude reduction of the adsorption capacity as the pH was elevated from 4 to 10. The presence of cosolvents had a negative effect on the process leading to substantial reductions in the adsor...

Modeling of the Association of Metal Ions with Heterogeneous Environmental Sorbents

AbstractThe use of mechanistic (surface-complexation, electric-double layer) and semi-empirical (affinity spectrum) models for representation of the association of metal ions with heterogeneous environmental materials, such as humic acids and soil particle surfaces, is compared. It is seen that mechanistic models are not nearly as mechanistic as one generally assumes, and that semi-empirical models are much more valuable than one might assume by comparison to simple Kd, models. A semi-empirical discrete-log-K-spectrum model was used to describe the binding of Co(II), as a function of pH and NaClO4 concentration, to two environmental substrates: leonardite humic acid and a kaolinitic subsoil. Excellent agreement of the model and the data was obtained over a wide range of solution composition. These models appear to be the most promising among several alternatives for modeling interactions of metal ions with complex heterogeneous environmental materials over a wide range of solution composition.